Coronavirus, shown under an electron microscope. It is part of a family of viruses that cause ailments including the common cold and Sars. Photograph: HPA/AP

A third member of one UK family has been infected by the new Sars-like virus that appears to have originated in the Middle East, according to the Health Protection Agency.

The first member of the family to fall ill had travelled to the Middle East and Pakistan. That person and a relative who had a pre-existing medical condition, which might have made them more susceptible to infection, were admitted to a Manchester hospital.

But the third family member to have contracted the novel coronavirus is said by the HPA to be recovering from a mild respiratory illness and is well. He or she has been advised not to meet with other people who are not part of the family, but only as a precaution. Other relatives and contacts of the latest person to be diagnosed are still being tracked down and tested.

There have been 12 confirmed cases worldwide, of which four were found in Britain. Three in Saudi Arabia and two in Jordan have died. But the HPA made it clear it did not consider the latest case to be an escalation of the problem.

Professor John Watson, head of the respiratory diseases department at the HPA, said: "Although this case appears to be due to person-to-person transmission, the risk of infection in contacts in most circumstances is still considered to be low. If novel coronavirus were more infectious, we would have expected to have seen a larger number of cases than we have seen since the first case was reported three months ago. However, this new development does justify the measures that were immediately put into place to prevent any further spread of infection and to identify and follow up contacts of known cases.

"We would like to emphasise that the risk associated with novel coronavirus to the general UK population remains very low. The HPA will continue to work closely with national and international health authorities and will share any further advice with health professionals and the public if and when more information becomes available."